How many Megabytes or Gigabytes do i get with unlimited data on a contract?

I want to go on this monthly contract plan: http://www.t-mobile.co.uk/...4m/extras/
It says i have unlimited data but i have been told that it isn't actually unlimited? If so how much data do i get?
And is it possible to cap my spend on this plan so i can't go over my contract?

Normally as much you can use. But the ISP can control their speed, and slow down your download speed.

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